Heavy Oil Recovery Method

Abstract

A hydrocarbon solvent such as benzene, platformate, or kerosene at a temperature in the range of 300.degree. to 700.degree. F. is injected into the top of tar sands at an injection well and forced through the formation to an adjacent production well. Injection of the solvent and production of oil are continued to maintain a gaseous phase across the top of the formation. The tar sand oil is made more mobile as a result of heating and dissolution of the solvent into the oil whereby the oil drains into the production well and is lifted to the surface.

Description

This invention relates to the production of oil and more particularly relates to the production of heavy, highly viscous oils from tar sands.

Enormous quantities of hydrocarbons exist in known deposits of high-density, high-viscosity oil in essentially unconsolidated formations referred to as tar sands. Examples of such formations are the Athabasca and Peace River tar sands in Canada Oil in such formations frequently includes substantial amounts of asphaltic materials that are insoluble in petroleum-naphthas. The oil may be highly viscous or even of a semisolid bituminous nature, or may have a high pour point. Any one of the properties alone may preclude recovery of oil from the tar sands by primary production methods.

Although the permeability of some of the tar sands is sometimes relatively high, because of the low mobility of the oil and the shallow depth of many tar sands which preclude applying high pressures to the tar sands without breaking through the overburden, effective production procedures have not been developed. The unconsolidated nature of the deposits prevents effective fracturing and propping of a fracture to form flow channels of high flow capacity and controlled location through the tar sand to production wells.

In-situ combustion has been attempted as a method of producing the tar sands, but has not been successful because the permeability of the tar sands to flow is sharply reduced by the three phases of gas, water, and oil ahead of the combustion front. Furthermore, the oil ahead of the combustion front remains cold and immobile. Attempts have been made to produce heavy oils from tar sand by by creating a permeable channel through tar sands from an injection well to a production well, and thereafter injecting air into the channel. It was believed that oil from the formation would diffuse into the channel burn and thereby heat the oil in the tar sands the increase its mobility. Such procedures have not been successful because the air injected largely bypasses the oil in the formation, flows into the production well, and ignites oil in the production well. As a result of all of these difficulties, some have gone to an expensive mining and surface treating of the tar sands to recover the hydrocarbons.

This invention resides in a process for producing low gravity, highly viscous oils from tar sands in which a hot hydrocarbon solvent such as benzene, platformate, and kerosene is injected into the top of the tar sands at an injection well and forced through the formation to an adjacent production well. The temperature of the injected hydrocarbons is high enough to maintain a gaseous phase to establish a permeable vapor-filled channel across the top of the formation. Oil flowing into the production well is lifted through the production at a rate maintaining a pressure not substantially greater than atmospheric pressure in the production well. As production continues, the upper portion of the tar sands is left filled with hydrocarbon vapors, or liquid of low viscosity formed by the condensation of hydrocarbon vapors, that can readily be recovered by a subsequent production step.

The single FIGURE of the drawings is a diagrammatic vertical sectional view through injection and production wells extending downwardly through a tar sand.

Referring to the drawing, a tar sand 10 is shown penetrated by an injection well indicated generally by reference numeral 12 and a production well spaced from the injection well and indicated generally by reference numeral 14. Both wells 12 and 14 are shown extending through an overburden 16 across the top of the tar sand 10, through the tar sand, and into an underlying formation 18.

The tar sand 10 is an incompetent or partially consolidated formation which can be parted by the injection of fluid at a high rate but cannot be fractured in the sense of creating a well-defined channel which can be propped open. This invention can be used when the permeability of the tar sand is relatively high, preferably exceeding 100 millidarcies, to permit flow at significant rates through the formation at low pressure differentials from the injection well to the production well. The mobility of the oil in tar sand 10 is low enough to prevent economically feasible production by primary production methods or by simple fluid drive processes. This invention is particularly advantageous in the recovery from tar sands of oil that includes substantial quantities of asphaltic materials that are insoluble in highly paraffinic hydrocarbons such as propane, butane or even light naphthas or condensate. This invention is particularly valuable when tar sand 10 is relatively shallow and cannot be subjected to high pressures, and, therefore, the pressure available for driving oil through the formation is limited.

Injection well 12 has casing 20 extending downwardly into the underlying formation 18. Casing 20 is cemented in place by conventional cementing procedures, after which perforations 22 through the casing and surrounding cement sheath are formed by any suitable method such as by use of shaped charges in the interval of the tar sand near its upper boundary. It is preferred that the perforations 22 be in the upper eighth of the tar sand 10. Casing 20 is capped at 24 and a tubing 26 run into the casing through a packer 28.

Production well 14 is spaced from the injection well 12 at a desirable distance, depending on the characteristics of the tar sand and the well pattern, to give suitable production rates. It is preferred that well 14 be at a distance less than about 600 feet from well 12 to give 5 to 10 acre spacing. Production well 14 has casing 30 extending downwardly through tar sand 10 into the underlying formation 18. Casing 30 is cemented in accordance with the conventional cementing procedures. After perforations 32 are formed in casing 30 and the surrounding cement sheath in the interval of tar sand 10 near its lower boundary, tubing 34 is run into well 14 and held in place at its lower end by a packer 36 set in casing 30 above the perforations. Tubing 34 extends through a cap 38 closing the upper end of the well 14. A pump 40 is anchored in the lower end of the tubing 34 for lifting oil through the production well.

In the method of this invention, a hot solvent is pumped down through tubing 26 and injected through the perforations 22 into the upper part of tar sand 10. The solvents should include hydrocarbons of low enough boiling point to result in a vapor-filled zone extending across the tar sands in the area that has been swept free of reservoir oil. The gaseous solvent will condense and dissolve in oil remaining in the vapor-filled zone and reduce its viscosity beyond the reduction resulting from heating alone. Any liquid of low viscosity that is capable of being vaporized at temperatures that will not cause cracking of the solvent or the tar sand oil and is miscible in the tar sand oil without precipitating constitutes in the oil can be used as the solvent. Preferred solvents because of their relatively low cost and good solvent properties are aromatic hydrocarbons or mixtures of hydrocarbons containing substantial amounts of aromatic hydrocarbons. Examples of such hydrocarbons are benzene, toluene, xylene, and highly aromatic mixtures of hydrocarbons having a boiling point range substantially the same as gasoline. Kerosene can also be used. Highly paraffinic hydrocarbons such as LPG, light naphthas or condensate are not suitable because of their inability to dissolve asphaltic constituents of the tar sand oil. Condensate composed principally of paraffinic hydrocarbons has been found to cause precipitation of asphalt from Athabasca or Peace River tar sands even though the condensate was approximately 70 percent C.sub.6 + and contained only about 7 percent C.sub.3 and C.sub.4.

The solvent is injected preferably in the vapor phase at a temperature in the range of 300.degree. to 700.degree. F. It is preferred that the temperature of the injected solvent be as high as possible without causing substantial cracking of the solvent as it passes down tubing 26 and into the tar sand 10. In any event, the temperature of the solvent should exceed its initial boiling point at the formation pressure in the tar sand. On entering the tar sand 10, solvent will remain in the vapor phase in the upper part of the tar sands where the reservoir has been flushed out. Where the solvent vapor contacts reservoir oil, the solvent will condense and dissolve in the oil in the tar sands. The raising of the temperature of the oil in the tar sands and the dilution of the oil by the hot solvent greatly increase the mobility of the oil in the tar sand 10. Continued injection of the hot solvent results in extension of a vapor-filled zone 42 across the top of the tar sand from well 12 to well 14. The single phase in the zone greatly increases the permeability of the formation to the injected solvent and allows circulation from the injection well to the production well at a low pressure differential.

During the injection of hot solvent at the injection well, oil is pumped from the production well by pump 40. Injection and production are continued after the solvent breaks through at the production well to heat the tar sands adjacent the production well and reduce the production well and reduce the pressure in the tar sands adjacent the production well to less than the vapor pressure of the solvent at the prevailing temperature. The maximum pressure which can be maintained in the reservoir adjacent the production well will, therefore, depend on the particular solvent used and the temperature of the solvent. If benzene is the solvent used and the temperature adjacent the production well is 400.degree. F., for example the pressure at the production well may be as high as 200 p.s.i. Higher pressures can be maintained if the temperature is increased. A low pressure, for example less than 100 p.s.i. and even as low as atmospheric pressure can be attained, adjacent the production well and is advantageous in increasing the volumes of the hot vapors and thereby increasing their effectiveness in driving oil from the formation.

Injection of hot solvent at well 12 and production of oil at well 14 is continued at a rate to extend a vapor phase zone 42 all of the way across the tar sand from the injection well to the production well. The vapors cone downwardly at the production well toward the perforations 32 and thereby cause the lower boundary of the vapor zone to slope downwardly. Oil adjacent well 14 drains by gravity drainage into the well through perforations 32. Oil in the tar sand 10 directly below zone 42 is heated and diluted by the hot solvent. The resultant mixture has a high mobility and flows along the lower boundary of zone 42, particularly along the downwardly sloping boundary of the production well 14, and drains into production well 14 through perforations 32. If part of the oil in the tar sand is vaporized it flows into zone 42 and is swept by the hot vaporized solvents to well 14. The downwardly flowing vapors mix with the heated oil near the production well and further reduce the viscosity of the oil flowing into the well.

The oil entering well 14 is lifted by pump 40 to the surface. Continued injection of hot solvent through well 12 and production of oil through well 14 cause zone 42 to be gradually enlarged downwardly. When the lower boundary of zone 42 approaches the lower boundary of the tar sand 10, or the viscosity of the remaining tar sand oil has been reduce by heating or dilution with solvent, the hot solvent injection is stopped. The solvent and low viscosity oil remaining in the tar sand can be readily displaced through the tar sand to the production well by a tertiary recovery process such as water or insert gas flooding. Forward combustion can also be used to displace the remaining oil.

It is essential to this invention that the hot solvent be injected into the formation near its upper boundary. If hot solvent is injected into a lower part of the tar sand, oil above the level of injection made fluid by the hot solvent drains downwardly into the vapor zone, condenses the hot solvent, and prevents the rapid establishment of a well-to-well vapor zone.

This invention has been described for a production well having perforations 32 near the lower boundary of the tars and 10 from the beginning of the operation. Perforations could initially be made near the upper boundary of the tar zone and then, as the zone 42 is enlarged, new perforations made at successively lower positions, in each instance below the lower boundary of the zone 42, and the packer 36 successively resets just above the lowest perforations. Oil made mobile or fluid by heat and dilution will still flow into the production well by gravity drainage, but will not have as far to drain.

Claims

I claim:

1. A method of producing heavy asphaltic oil from a tar sand comprising injecting into the upper portion of the tar sand at an injection well penetrating the tars and a hot fluid consisting essentially of a hydrocarbon solvent selected from the group consisting of at least one aromatic hydrocarbon selected from the group consisting of benzene, toluene and xylene, mixtures containing a high concentration of such aromatic hydrocarbons with nonaromatic hydrocarbons of of substantially the same boiling point, and kerosene; the temperature of the hot solvent injected into the formation being in the range of 300.degree. to 700.degree. F. such that the temperature exceeds the initial boiling point of the solvent at the tar sand formation pressure to form a vapor zone extending substantially horizontally across the tar sad near the lower boundary thereof at a production well production well spaced from the injection well; and continuing the injection of the hot solvent to extend the vapor zone in the tar sand to the vicinity of the production well and heat oil in the tar sand below the vapor zone whereby said heated oil whereby said heated oil drains to the production well.